$ heroku addons:create heroku-postgresql
Creating HEROKU_POSTGRESQL_INSTANCE... done, (free)
Adding HEROKU_POSTGRESQL_INSTANCE to strange-turtle-1234... done
Setting DATABASE_URL and restarting strange-turtle-1234... done, v3
Database has been created and is available
! This database is empty. If upgrading, you can transfer
! data from another database with pgbackups:restore
Use `heroku addons:docs heroku-postgresql` to view documentation.

Promote the database (replace HEROKU_POSTGRESQL_INSTANCE with the name from the above output)

WordPress will prompt for updating the database. After that you'll be good
to go.

Deployment optimisation

If you have files that you want tracked in your repo, but do not need deploying (for example, *.md, *.pdf, *.zip). Then add path or linux file match to the .slugignore file & these will not be deployed.